The aggregate used in concrete makes up a material portion of the weight of the concrete. To reduce the weight of the concrete, various porous or expanded aggregate, such as expanded clay and shale, have been used. For example, concrete made with lightweight aggregate may weigh about 100 pounds per cubic foot, whereas regular concrete blended in the same way as lightweight concrete may weigh about 140 pounds per cubic foot. Volcanic cinders, which are commonly referred to as scoria, are highly porous and have also been used. Lightweight concrete has numerous potential advantages, including the reduction in the weight of the concrete structure, reduced energy cost in pumping the concrete and strength-to-weight advantages.
Unfortunately, lightweight concrete has not achieved all of its potential advantages due primarily to the absorption of water and cement in copious amounts by the porous aggregate. The absorption rate of the porous aggregate is, of course, significantly increased when the concrete is pumped due to the high hydrostatic pressure of the pumping system. The absorption of cement is costly and wasteful and makes pumping of the concrete difficult or impossible. Thus, the porosity of the aggregate which is the source of the potential advantages is also the source of the primary problem which has prevented the attainment of these advantages. Moreover, shale is expanded in a process which consumes vast amounts of energy, mainly in the form of natural gas.
Scoria is porous in its natural state and does not require energy expenditures for its porosity. However, it is subject to the water-cement absorption problem noted above. As alluded to in U.S. Pat. No. 4,095,995 issued to Ullrich, it is known to prevent the escape of water into porous aggregate by sealing the particles of the aggregate with a thermoplastic coating or by using highly water repellent sprays, such as silicone resin solutions. However, this deteriorates the bond between the aggregate and the cement which is required for adequate strength of the concrete. It is essential that any method for reducing absorption must not materially adversely affect the ultimate strength of the concrete.